P.T. Mathiopoulos

ADC and DSP challenges in the development of software radio base stations

The need for software-defined radios raises a number of technical challenges, which play a significant role in the development of third-generation personal communications systems (PCS). Some of the most important technical challenges deal with analog-to-digital conversion and digital signal processing technologies, which, in many cases, cannot provide the advanced hardware needed to support the demanding PCS telecommunications services. We discuss these two technical challenges, focusing primarily on base station radio systems. We first identify the most important requirements for ADC and DSP technologies, and we then extensively discuss and/or propose enabling schemes that could relax these requirements and aid the implementation of software radio base stations. Furthermore, we discuss new structures that can support the development of physically distributed base stations.

Optimal detection of coded differentially encoded QAM and PSK signals with diversity reception in correlated fast Rician fading channels

The optimal sequence estimator for digital signals received over Lambda different channels is derived. Each of these channels corrupts the transmitted signal by a mixture of additive white Gaussian noise (AWGN) and frequency-nonselective, correlated, fast Rician fading. By analysis it is shown that for the lth (1 >

A comparison study of the uplink performance of W-CDMA and OFDM for mobile multimedia communications via LEO satellites

This article presents a comparison study of the uplink performance of two well-known transmission schemes, W-CDMA and OFDM, for mobile multimedia communications via LEO satellite. In order to provide a framework for a fair comparison we have considered for both schemes common parameters, including identical channel bandwidth, coding/decoding algorithms and similar net data rate. The values of these parameters have been appropriately selected in accordance with the corresponding UMTS system specifications. For W-CDMA, we propose a receiver structure that eliminates ISI for both channel estimation and equalization at the LEO satellite. For OFDM, we propose a system the design of of which is based upon the parameters of W-CDMA for UMTS. For both schemes appropriate transceiver structures are proposed and analyzed and their performance is evaluated in terms of computer simulation. The design of these transceiver structures effectively takes into account the statistical characteristics of the LEO satellite channel, so that the overall system performance is improved. The simulation results evaluating the uplink BER performance for a single user indicate that, although the BER performance of W-CDMA is slightly better than that of OFDM, the performance differences are rather small. Furthermore, these differences in performance become even smaller when channel coding is applied and when LOS conditions between the mobile terminal and the LEO satellite exist. Since these results have not shown a clear-cut winner, further comparative studies are required in order to identify the most appropriate transmission scheme for applications supporting global mobile multimedia communication via LEO satellite networks. Such studies should include determination of the downlink BER performance as well as spectrum efficiency and capacity performance equations for both up and down satellite communication links.

Analytical level crossing rates and average fade durations for diversity techniques in Nakagami fading channels

Using a unified methodology, we derive analytical expressions for the level crossing rates (LCR) and average fade durations (AFD) for three diversity reception techniques and a general Nakagami fading channel. We provide novel analytical expressions for selection combining (SC) and equal-gain combining (EGC), and rederive, in a more general manner, the case of maximal-ratio combining (MRC). Our general results reduce to some specific cases previously published. These results are used to examine the effects on the concerned quantities of the diversity technique, the number of receiving branches and the severity of fading. It is observed that as the Nakagami m-parameter and the diversity order increase, the behavior of the combined received envelope for EGC follows closely the one for MRC, and distances itself from SC.

Analysis and performance evaluation of dynamic channel reservation techniques for LEO mobile satellite systems

This paper deals with the performance analysis and evaluation of an efficient dynamic channel reservation (DCR) scheme that is suitable for low Earth orbit mobile satellite systems. The proposed DCR scheme makes the channel reservation decision according to the expected number of handover requests and the positions of the mobile terminals so as to effectively reduce the handover failure probability. The semi-analytical and computer simulation results attained show that the proposed DCR scheme can effectively reduce the handover failure probability as compared to the fixed channel reservation scheme (FCR).

Reverse link capacity analysis for cellular CDMA systems employing successive interference cancellation

In this paper, a path-loss-based successive interference cancellation (SIC), which successively decodes mobile users assigned to the same cell from the one with minimum path loss to the one with the maximum path loss and removes their contribution from the received signals in the same sequence, is proposed to reduce both the outage probability and the inter-cell interference of cellular CDMA systems. Without excluding the effects of lognormal shadowing fading and for an arbitrary cluster of homogenous hexagonal cells, we have theoretically analyzed the reverse link capacity improvement brought by the path-loss-based SIC over a conventional matched filter. Furthermore, by taking a pedestrian application in cdma2000 as an example, computer simulations are performed, and the obtained simulation results have perfectly confirmed our theoretical analysis.

Neural-net-based receiver structures for single- and multiamplitude bandlimited signals in CCI and ACI channels

This paper presents analysis and performance-evaluation results for several neural-network-based nondecision-feedback receiver structures, which improve the performance of bandlimited single- and multiamplitude signals transmitted over additive interference channels, such as cochannel interference (CCI) and adjacent channel interference (ACI). In particular, we propose, analyze, and evaluate a training algorithm for Nyquist-filtered single- and multiamplitude signals, based upon a novel nonuniform signal-sampling technique. We also introduce a novel nonlinear activation function for multiamplitude signals and evaluate its performance via computer simulation and in conjunction with various bandlimited signaling formats, detection techniques, and neural-network structures. Bit-error rate (BER) performance-evaluation results of the proposed neural-network receivers for coherent and noncoherent detection of Nyquist- and Butterworth-filtered single- and multiamplitude signals have shown performance improvements in the presence of CCI and ACI.

A new approach for mitigating A/D conversion requirements in software radio base stations

In this paper we present a novel method, which significantly relaxes the stringent ADC requirements encountered in software radio applications. By digitizing a residual signal, instead of the original received wideband signal, we show that the high dynamic signal range at the input of the ADC can be significantly reduced. Such reduction is accomplished by predicting the largest power signal and subsequently subtracting a portion of it from the overall received signal. Employing AR and PAR prediction algorithms, using several well-known modulation schemes, we obtain various computer-simulated performance evaluation results, which justify the effectiveness of the proposed methodology.

Reverse link inter-cell interference analysis for cellular CDMA systems with controlled power disparities

When certain new techniques, such as, for example, combining macrodiversity and multiuser detection by successive interference cancellation, are employed in the base stations of cellular CDMA systems, the received power of the mobile users assigned to the same cell has controlled disparities. For such kinds of cellular CDMA systems, their reverse link inter-cell interference can no longer be analyzed with the conventional inter-cell interference model based upon the assumption of equal received power. We propose a new inter-cell interference model to analyze the reverse link inter-cell interference of the cellular CDMA system with controlled power disparities. Based upon the new model, we derive analytical solutions for the reverse link inter-cell interference and verify the validity of the analytical solutions by computer simulations.

Performance of multicode DS/CDMA with noncoherent M-ary orthogonal modulation in multipath fading channels

This paper presents the performance analysis and simulation of a multicode DS/CDMA system with noncoherent M-ary modulation, operating over a Rayleigh or Nakagami fading environment. This type of transceiver is specified for the reverse link of the IS-95B and cdma2000 (Radio configurations 1 and 2) systems, and is intended to serve high-rate applications such as data transfer and video communications. After a statistical characterization of the interference terms, we make use of the Gaussian approximation (GA) in order to obtain the bit error rate (BER). However, unlike other analyses relying on the GA, in our derivation we take into account the fact that all the codes transmitted by a mobile user fade in unison. As demonstrated via computer simulations, this fact is crucial to obtain a reliable estimate of the BER, especially when equal-gain combining (EGC) is used at the receiver.